Liquid electrophotographic printing, also referred to as liquid electrostatic printing, uses liquid toner to form images on a print medium. A liquid electrophotographic printer may use digitally controlled lasers to create a latent image in the charged surface of an imaging element such as a photo imaging plate (PIP). In this process, a uniform static electric charge is applied to the photo imaging plate and the lasers dissipate charge in certain areas creating the latent image in the form of an invisible electrostatic charge pattern conforming to one colour separation of the image to be printed. An electrically charged printing substance, in the form of liquid toner, is then applied and attracted to the partially-charged surface of the photo imaging plate, recreating a separation of the desired image.
In certain liquid electrophotographic printers, a transfer member, such as an intermediate transfer member (ITM) is used to transfer developed liquid toner to a print medium. For example, a developed image, comprising liquid toner aligned according to a latent image, may be transferred from a photo imaging plate to a transfer blanket of an intermediate transfer member. This transfer occurs via predominantly electrical and mechanical forces that exist between the charged liquid toner and the intermediate transfer member which is often biased at a particular voltage level. Pure mechanical force, using zero electrical potential difference between the blanket of the intermediate transfer member and liquid toner produces poor print quality. From the intermediate transfer member, the toner is transferred to a desired substrate, which is placed into contact with the transfer blanket.
At least two different methodologies may be used to print multi-color images on a liquid electrophotographic printer. These involve the generation of multiple separations, where each separation is a single-color partial image. When these separations are superimposed, they result in the desired full color image being formed. In a first methodology, a color separation layer is generated on the photo imaging plate, transferred to the intermediate transfer member and is finally transferred to a substrate. Subsequent color separation layers are similarly formed and are successively transferred to the substrate on top of the previous layer(s). This is sometimes known as a “multi-shot color” imaging sequence. In a second methodology, a “one shot color” process is used. In these systems, the photo imaging plate transfers a succession of separations to the transfer blanket on the intermediate transfer member, building up each separation layer on the blanket. Once some number of separations are formed on the transfer blanket, they are all transferred to the substrate together.